The present invention relates to brush mechanisms for rotating electrical machines with a slip ring; and more particularly, the invention relates to a brush mechanism for a homopolar generator.
Homopolar generators (HPGs) are pulsed, rotating electrical machines which are being developed as pulsed power supplies for applications that include electromagnetic propulsion, welding, and fusion devices. These machines are inherently low voltage, very high current generators.
Collecting and transferring the very large currents from high surface speed slip rings is the most demanding task in an HPG. Brushes of sintered copper graphite are typically used to make contact with the rotating slip ring surface. These brushes are silver brazed to a brush strap that carries the current to a current collector ring. The brush straps must conduct extremely high currents without becoming excessively hot. They must operate in a relatively high magnetic field and thus are subjected to high electromagnetic forces during a current pulse. The strap must provide sufficient elastic spring force to lift the brush clear of the slip ring. Finally, and most important, the strap must provide a dynamically stable brush mount sufficiently soft radially to allow the brush to track the slip ring but sufficiently stiff axially and circumferentially to ensure that the brush returns to exactly the same orientation on the rotor after each actuation.
As brush material wears away with use, it is desirable to compensate for this wear so that the brush is retracted the same distance from the slip ring surface throughout its useful lifetime, ensuring consistent brush actuation times and down forces. Unfortunately for compact, high packing factor brush assemblies that require short brush straps, the bending stress in the solid copper strap is exceeded during brush actuation, causing the brush strap to yield in the "brush down" position and resulting in a loss of ability to retract the brush. Conventionally hardened copper (hardened by cold working or rolling) cannot be used to raise the yield strength of the brush strap since it will be annealed during the process of brazing the brush strap to the brush.
Another problem with prior art brush mechanisms is that the discharge current in the brush straps react with each other and with the excitation magnetic field to lift the brushes off the slip ring during a discharge.